Police and prosecutors were strongly criticised today for pursuing the case against two teenagers who were cleared of plotting a Columbine-style massacre at their own school.

A jury took just 45 minutes to clear Matthew Swift, 18, and Ross McKnight, 16, of planning to murder teachers and pupils at Audenshaw High School in Greater Manchester.

Following the verdicts, the barrister who defended one of the youngsters said it was an "unnecessary, heavy-handed prosecution" and an expensive waste of public money.

McKnight's father Ray, a serving police officer, said both his son and Swift had gone through "purgatory" and "absolute agony" after spending six months remanded in custody.

The prosecution had claimed that the teenagers had been plotting a gun rampage similar to that carried out by Eric Harris and Dylan Klebold, who murdered 12 students and a teacher at a school in Columbine in 1999.

Much of the case was based on the pair's journals and diaries, which contained tirades against the school and society and included maps and plans of the school buildings.

Both Harris and Klebold had kept similar documents before their attack.

The police were so convinced of the pair's guilt that they decided to fly two detectives to Colorado ahead of the trial to question the homicide department that investigated the Columbine killings.

The Columbine lead investigator, Kate Battan, was then flown to Manchester and was mooted as a possible prosecution witness, but she was never called to give evidence.

Swift and McKnight were arrested in March, a month before prosecutors claimed they had planned to carry out the alleged attack.

The prosecution said the plan, which the pair had codenamed Project Rainbow, was to have been carried out on 20 April – the 10th anniversary of the Columbine atrocity.

It was also alleged Swift had tried to obtain explosives and was researching how to make firearms at home.

The defence told the jury the journals were the scribblings of two teenage boys with overactive imaginations, and that their thoughts and writing had never gone beyond "fantasy".

Ray McKnight told the court his son was full of "hare-brained" schemes.

Both teenagers were cleared of conspiracy to murder and conspiracy to cause explosions likely to endanger life or cause serious injury to property between November 2007 and 15 March this year following a three-week trial.

Swift fought back tears as the foreman of the jury delivered the verdicts and breathed a sigh of relief as he left the dock. McKnight showed no emotion, standing with his head bowed.

The alleged plot came to light only after McKnight, high on drink and drugs, made a late-night phone call to a girl he was fond of in which he allegedly "confessed" to the plan. The girl told her mother, and police were called.

Officers found the journals and diaries during raids at the teenagers' homes in Denton, Greater Manchester, on 14 March.

McKnight, who referred to himself by the nickname 72, wrote a diary entry reading: "72 + Swift = God and s***loads of dead people."

He went on to talk about the "greatest massacre ever" and killing thousands of people.

Officers found a manual entitled the Anarchist Cookbook at Swift's home. It contained instructions on ingredients to use in explosives, how chemicals could be converted into explosives, and chapters on lethal weapons, boobytraps and Molotov cocktails.

They recovered a safe from Swift's bedroom which contained plans of the school and details of how to use acetone peroxide as a detonator.

Last week Swift told the jury his diary entries were "naive and pathetic ways to channel my teenage angst". He said: "I was 16 with a vivid imagination."

His bookshelves contained Hitler's autobiography, Mein Kampf, and the Turner Diaries, said to have inspired the Columbine killers. Police also found two films about school shootings – Elephant and Zero Day – at his home.

Six mobile phone video clips of Swift and McKnight allegedly experimenting with explosives were shown to the jury, but no explosives were found during police searches. Swift said his interest in Columbine had begun "in the lower sixth after we were shown the film Bowling for Columbine.

"At the time I felt the lads might be misunderstood or there might be some other reason why they did it," he said. After watching the film, he said, he became interested in Harris's character.

Asked by his barrister, Stephen Riordan QC, whether he was sympathetic to Harris, Swift replied: "In a way." He said McKnight "felt the same as well".

He told the court he had been prescribed antidepressants in January, but had only taken three of the tablets.

He denied ever buying chemicals from shops or online that could be used to make explosives, but admitted being curious about explosives and owning the Anarchist Cookbook.

Asked whether he or McKnight had intended to kill, Swift replied: "No" and said they had not intended to plant explosives to endanger life.